Efficient oxidation of oleanolic acid derivatives using magnesium bis(monoperoxyphthalate) hexahydrate (MMPP): A convenient 2-step procedure towards 12-oxo-28-carboxylic acid derivatives

Article abstract of DOI:10.3762/bjoc.8.17

A new straightforward and high yielding procedure to convert oleanolic acid derivatives into the corresponding δ-hydroxy-γ- lactones, by using the convenient oxidizing agent magnesium bis(monoperoxyphthalate) hexahydrate (MMPP) in refluxing acetonitrile, is reported. In addition, a two-step procedure for the preparation of oleanolic 12-oxo-28-carboxylic acid derivatives directly from Δ12-oleananes, without the need for an intermediary work-up, and keeping the same reaction solvent in both steps, is described as applied to the synthesis of 3,12-dioxoolean-28-oic acid.

Full text of DOI:10.3762/bjoc.8.17

Efficient oxidation of oleanolic acid derivatives using
magnesium bis(monoperoxyphthalate) hexahydrate
(MMPP): A convenient 2-step procedure towards
12-oxo-28-carboxylic acid derivatives
Jorge A. R. Salvador*1,2, Vânia M. Moreira1,3, Rui M. A. Pinto1, Ana S. Leal1
and José A. Paixão4
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 164–169.
1Grupo de Química Farmacêutica, Faculdade de Farmácia da
Universidade de Coimbra, Pólo das Ciências da Saúde, Azinhaga de
Santa Comba, 3000-548, Coimbra, Portugal, 2Centro de
Neurociências e Biologia Celular, Universidade de Coimbra,
3004-517 Coimbra, Portugal, 3Centro de Química de Coimbra,
Faculdade de Ciências e Tecnologia da Universidade de Coimbra,
Rua Larga, 3004-535 Coimbra, Portugal and 4CEMDRX,
Departamento de Física, Faculdade de Ciências e Tecnologia,
Universidade de Coimbra, 3004-516 Coimbra, Portugal
doi:10.3762/bjoc.8.17
Received: 25 November 2011
Accepted: 04 January 2012
Published: 30 January 2012
Associate Editor: C. Stephenson
© 2012 Salvador et al; licensee Beilstein-Institut.
License and terms: see end of document.
Email:
Jorge A. R. Salvador* - salvador@ci.uc.pt
* Corresponding author
Keywords:
bismuth(III) triflate; δ-hydroxy-γ-lactones; MMPP; oleanolic acid;
triterpenoid
Abstract
A new, straightforward and high yielding procedure to convert oleanolic acid derivatives into the corresponding δ-hydroxy-γ-
lactones, by using the convenient oxidizing agent magnesium bis(monoperoxyphthalate) hexahydrate (MMPP) in refluxing aceto-
nitrile, is reported. In addition, a two-step procedure for the preparation of oleanolic 12-oxo-28-carboxylic acid derivatives directly
from Δ12-oleananes, without the need for an intermediary work-up, and keeping the same reaction solvent in both steps, is
described as applied to the synthesis of 3,12-dioxoolean-28-oic acid.
Findings
The molecular diversity that arises from research into natural drug development [3]. The chemistry of oleanane-type triter-
products represents a valuable tool for driving drug discovery penoids has been investigated with particular interest and many
and development [1,2]. In this context, pentacyclic triter- relevant biological and pharmacological activities of these
penoids are currently regarded as important scaffolds for new derivatives have been reported in the literature, among which
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Beilstein J. Org. Chem. 2012, 8, 164–169.
are antitumor, antiviral, anti-inflammatory, hepatoprotective, We found that the reaction of 3-oxooleanolic acid 1 with
gastroprotective, antimicrobial, antidiabetic, and hemolytic 2.0 equiv of MMPP, in refluxing acetonitrile, afforded the
properties, as well as many others [3-5]. Functionalized corresponding δ-hydroxy-γ-lactone 2 in 85% yield after 5 hours
γ-lactones are important building blocks of bioactive natural (Table 1, entry 1). These new reaction conditions were success-
products [6,7]. The δ-hydroxy-γ-lactone motif is part of such fully extended to OA 3 and other 3β-substituted OA derivatives
bioactive natural products as (±)-muricatacin [8,9] or the three 5, 7 and 9 (Table 1, entries 2–5).
hydroxylactones found in the mushroom Mycoleptodonoides
aitchisonii [10]. Terpenoid δ-hydroxy-γ-spirolactones have The substrates were dissolved in acetonitrile under reflux, and
been found to act as significant feeding deterrents to the lesser MMPP (2.0–3.0 equiv) was added to the solution under strong
mealworm Alphitobius diaperinus [11]. In particular, oleanane- magnetic stirring. After completion of the reaction, the magne-
type triterpenoids bearing a γ-lactone function, either isolated sium salts were easily filtered off after evaporation of the aceto-
from natural sources or obtained by semisynthesis, have shown nitrile and suspension of the resulting white solid in ethyl
interesting biological activities [12-14]. From the synthetic acetate. We found that 2.0 equiv of MMPP were sufficient to
point of view, the oxidative 28,13β-lactonization allows the effectively convert 3-oxooleanolic acid 1, OA 3, and
preparation of 12α-hydroxyoleananes with a protected 3β-acetoxyoleanolic acid 5 into the corresponding δ-hydroxy-γ-
28-carboxyl acid function. In fact, 12α-hydroxy-3-oxooleanan- lactones 2, 4 and 6, in 84 to 88% yield (Table 1, entries 1–3).
28,13β-olide (2) is a key intermediate in the synthesis of Substrates 7 and 9, bearing a trifluoroacetoxy and a methoxy
S-0139, an endothelin A receptor antagonist [15]. Moreover, as group at C3, respectively, required higher amounts of the
part of our ongoing work on pentacyclic triterpenoid chemistry reagent and longer reaction times (Table 1, entries 4 and 5). The
[16,17], we recently demonstrated that oleanolic δ-hydroxy-γ- formation of the oleanolic δ-hydroxy-γ-lactones 2, 4, 6, 8 and
lactones can be efficiently converted into the corresponding 10, may be explained by epoxidation of the parent Δ12-oleanane
12-oxo-28-carboxylic acid derivatives by bismuth(III) triflate compound, followed by nucleophilic attack of the 28-carboxyl
catalysis [18]. This new approach not only avoids an inconve- group at C13 from the β-face, with ring-opening of the 12α,13α-
nient multistep synthesis by means of a protection/deprotection epoxide intermediate [15,44].
strategy [19,20] but also results in chemical modification of ring
C, a strategy known to increase the anti-inflammatory and cyto- Quite recently, we demonstrated that the 28,13β-lactonization
toxic activities of oleanolic acid (OA) derivatives [19,21,22].
of 3-oxooleanolic acid 1 promoted by Bi(OTf)3·xH2O affords a
3-oxo-18α-olean-28,13β-olide product, with inversion of con-
Oleanolic δ-hydroxy-γ-lactones can be obtained from Δ12- figuration at the C18-stereocenter, as demonstrated by X-ray
oleananes by oxidative 28,13β-lactonization. This reaction was crystallography [45,46]. In order to assign the orientation
performed under photochemical irradiation [23,24], but weak of the 18-H of the 12α-hydroxy-γ-lactones obtained in this
selectivity and low isolated yields were observed. Alternatively, work, 2D NMR data were collected for compounds 2, 6
oxidation reagents such as H2O2 in acetic acid [25,26], the in- and 8, and X-ray data were gathered for compound 4.
organic salt mixture KMnO4/CuSO4 [27], ozone [15,28,29] and Combining 1D and 2D-NMR spectroscopy, we were able to
m-chloroperoxybenzoic acid (mCPBA) [30,31] have also been determine the chemical shift of 18-H (2.02 ppm) for compound
reported. Magnesium bis(monoperoxyphthalate) hexahydrate 2. This value is much lower than the one of the parent substrate
(MMPP) is commercially available, inexpensive and relatively 1 (2.84 ppm), which may be explained by magnetic anisotropy
stable [32-34] and has been used in the oxidation of various induced by the 28,13β-lactone moiety. It is also interesting to
functional groups [35-42]. This oxidant is non-shock-sensitive note that a long-distance coupling between the 18-H and 12β-H
and non-deflagrating [43]. Moreover, its use greatly simplifies (3.90 ppm) signals was found in the COSY spectrum of 2.
the isolation of the reaction products, because it may simply be Correlation between these two signals was also observed in the
filtered off from the reaction crude, which is then worked up as NOESY experiment and, therefore, the β-configuration was
usual.
assigned at the C18-stereocenter. The same NMR pattern was
present for compounds 6 and 8. Unequivocal evidence of the
In this letter, we report the use of MMPP for the efficient and molecular structure of compound 4 was obtained by single-
high-yielding oxidation of OA derivatives to afford the corres- crystal X-ray crystallography, and the ORTEP diagram with the
ponding δ-hydroxy-γ-lactones. Moreover, we have set up a corresponding atomic numbering scheme is depicted in
protocol that allows the convenient sequential two-step Figure 1.
preparation of 3,12-dioxoolean-28-oic acid directly from
3-oxooleanolic acid, without the need of an intermediary work- In the past few years, bismuth(III) salts have emerged as con-
up, and keeping the same reaction solvent in both steps.
venient reagents for the development of new chemical processes
165

Beilstein J. Org. Chem. 2012, 8, 164–169.
Table 1: Reaction of OA derivatives with MMPP to afford oleanolic δ-hydroxy-γ-lactones directly.a
Entry
Substrate (mmol)
MMPP (equiv) Time (h)
Productb
Yieldc (%)
1
2.0
2.0
2.0
2.6
3.0
5
85
1 (0.16 mmol)
2
4
6
2
3
4
5
24
84
88
98
92
3 (0.16 mmol)
8
5 (0.15 mmol)
24
7 (0.13 mmol)
8
24
10
9 (0.16 mmol)
aReactions were performed in acetonitrile, under reflux; bAnalytical data for compounds 2 [29], 4 [28], 6 [27], 8 [18] and 10 [18] are in accordance with
the literature; cIsolated yield.
under more “ecofriendly” reaction conditions, which avoid the performed in acetonitrile, we designed a protocol to perform the
use of large amounts of toxic and corrosive materials [47-51]. synthesis of oleanolic 12-oxo-28-carboxylic acid derivatives
Bearing in mind the solubility properties of MMPP and that directly from Δ12-oleananes, without the need for an inter-
both the oxidative 28,13β-lactonization and the bismuth(III) mediary work-up, and keeping the same reaction solvent in both
triflate-catalyzed direct opening of δ-hydroxy-γ-lactones are steps (Scheme 1).
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Beilstein J. Org. Chem. 2012, 8, 164–169.
Figure 1: ORTEP diagram of compound 4 (50% probability level, H atoms of arbitrary sizes). The asymmetric unit also contains a molecule of
CH3CN.
Scheme 1: Sequential 2-step synthesis of 3,12-dioxoolean-28-oic acid (11) directly from 3-oxooleanolic acid (1).
Thus, after the formation of the 12α-hydroxy-28,13β-olide the in situ generation of a Brønsted acid species from
compound 2 by MMPP oxidation, a filtration step allowed bismuth(III) triflate, which promotes ring opening
the elimination of insoluble magnesium salts, taking of the 28,13β-olide group, creating a tertiary carbocation at
advantage of their low solubility in acetonitrile. Then, a C-13. Then, a concerted stereoselective 1,2-migration of the
catalytic amount of bismuth(III) triflate (5 mol %) was added 12β-H to the 13β-position with the rearrangement of the 12α-
to the resulting filtrate, and the expected 12-oxo-28-carboxylic hydroxy group affords the final 12-oxo-28-carboxylic acid
acid 11 was obtained, in 85% yield, after the typical structure [18]. The molecular structure of compound 11, deter-
work-up procedure [18]. The formation of compound 11 mined by single-crystal X-ray crystallography, is shown in
from the 12α-hydroxy-28,13β-olide 2 is likely to occur due to Figure 2.
Figure 2: ORTEP diagram of compound 11 (50% probability level, H atoms of arbitrary sizes).
167

Beilstein J. Org. Chem. 2012, 8, 164–169.
3. Salvador, J. A. R., Ed. Pentacyclic Triterpenes as Promising Agents in
Cancer; Nova Science Publishers, Inc.: New York, 2010.
ISBN 978-1-60876-973-5.
In conclusion, we have found a new straightforward procedure
to convert OA derivatives into δ-hydroxy-γ-lactones, in very
high yields, using the convenient oxidizing agent MMPP. This
procedure has considerable advantages over the previously
reported oxidation methods, because no other positions of the
molecule are oxidized concomitantly, it avoids the use of halo-
genated solvents, and allows easy recovery of the reaction prod-
ucts. Combination of this oxidative 28,13β-lactonization
process with the ability of bismuth(III) triflate to catalyze the
opening of the resulting δ-hydroxy-γ-lactone with subsequent
generation of the carbonyl group, allowed us to set up a sequen-
tial two step strategy for the preparation of 3,12-dioxoolean-28-
oic acid (11) directly from 3-oxooleanolic acid 1, that avoids an
intermediary work-up and conveniently uses the same reaction
solvent in both steps. Thus, the procedure reported herein
greatly simplifies the obtainment of oleanolic δ-hydroxy-γ-
lactones, which are versatile intermediates for organic syn-
thesis, and in addition can provide very easy access to the
corresponding oleanolic 12-oxo-28-carboxylic acids.
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Supporting Information
The Supporting Information contains the typical procedure
for the MMPP oxidative 28,13β-lactonization and
preparation of compounds 2, 4, 6, 8 and 10. Moreover, the
procedure for the sequential two step synthesis of
3,12-dioxoolean-28-oic acid (11) is described and the 1D
and 2D NMR spectra of compounds 2, 4, 6, 8, 10 and
1D NMR spectra of compound 11 are shown.
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Supporting Information File 1
Experimental and analytical data.
doi:10.1016/j.bmc.2009.07.050
[http://www.beilstein-journals.org/bjoc/content/
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Acknowledgements
Jorge A. R. Salvador wishes to thank Universidade de Coimbra
for financial support. Vânia M. Moreira wishes to thank
Fundação para a Ciência e a Tecnologia for financial support
(SFRH/BPD/45037/2008). Ana S. Leal wishes to thank
Fundação para a Ciência e a Tecnologia for financial support
(SFRH/BD/41566/2007).
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